A preview apparatus in a single lens reflex camera is provided in which a mirror drive mechanism, a diaphragm drive mechanism, and a shutter are driven in association with the rotation of a single-revolution cam gear which is driven by a motor. When the release switch is turned ON, the one-turn cam gear is rotated by single-revolution in the forward direction, and when an external preview switch is turned ON, the preview operation is carried out by the reverse rotation of the single-revolution cam gear within a predetermined angle range in which the stop-down operation takes place without causing an upward movement of the mirror.